Conductive Thick Film Paste For Solar Cell Contacts

ABSTRACT

The present invention relates to an inorganic reaction system used in the manufacture of electroconductive pastes. The inorganic reaction system comprises a lead containing matrix forming composition and a tellurium oxide additive. Preferably the lead containing matrix forming composition is between 5-95 wt. % of the inorganic reaction system, and the tellurium oxide additive is between 5-95 wt. % of the inorganic reaction system. The lead containing matrix forming composition may be a glass frit, and may comprise lead oxide. Another aspect of the present invention relates to an electroconductive paste composition that comprises metallic particles, an inorganic reaction system as previously disclosed, and an organic vehicle. Another aspect of the present invention relates to an organic vehicle that comprises one or more of a binder, a surfactant, a solvent, and a thixatropic agent. Another aspect of the present invention relates to a solar cell printed with an electroconductive paste composition as disclosed, as well as an assembled solar cell module. Another aspect of the present invention relates to a method of producing a solar cell.

FIELD OF THE INVENTION

The present invention relates to electroconductive pastes as utilized insolar panel technology. Specifically, in one aspect, the presentinvention relates to an inorganic reaction system for use inelectroconductive pastes. Another aspect of the present inventionrelates to an electroconductive paste composition comprising aconductive metal component, an inorganic reaction system, and an organicvehicle. Another aspect of the present invention relates to a solar cellproduced by applying an electroconductive paste, which comprises anelectroconductive metal, an inorganic reaction system, and an organicvehicle, to a silicon wafer. Yet another aspect of the present inventionrelates to a solar cell module assembled using solar cells produced byapplying an electroconductive paste to a silicon wafer, wherein theelectroconductive paste comprises an electroconductive metal, aninorganic reaction system, and an organic vehicle.

BACKGROUND OF THE INVENTION

Solar cells are devices that convert the energy of light intoelectricity using the photovoltaic effect. Solar power is an attractivegreen energy source because it is sustainable and produces onlynon-polluting by-products. Accordingly, a great deal of research iscurrently being devoted to developing solar cells with enhancedefficiency while continuously lowering material and manufacturing costs.When light hits a solar cell, a fraction of the incident light isreflected by the surface and the remainder transmitted into the solarcell. The transmitted light/photons are absorbed by the solar cell,which is usually made of a semiconducting material, such as silicon. Theabsorbed photon energy excites electrons from the atoms of thesemiconducting material, generating electron-hole pairs. Theseelectron-hole pairs are then separated by p-n junctions and collected byconductive electrodes that are applied on the solar cell surface.

The most common solar cells are those based on silicon, moreparticularly, a p-n junction made from silicon by applying a dopantdiffusion layer onto silicon substrate, coupled with two electricalcontact layers or electrodes. In a p-type semiconductor, dopant atomsare added to the semiconductor in order to increase the number of freecharge carriers (positive holes). Essentially, the doping material takesaway weakly bound outer electrons from the semiconductor atoms. Thepurpose of p-type doping is to create an abundance of holes. In the caseof silicon, a trivalent atom is substituted into the crystal lattice.One example of a p-type semiconductor is silicon with a boron oraluminum dopant. Solar cells can also be made from n-typesemiconductors. In an n-type semiconductor, the dopant atoms provideextra electrons to the host substrate, creating an excess of negativeelectron charge carriers. Doping atoms, donors, usually have one morevalence electron than one type of the host atoms. The most commonexample is atomic substitution in group IV solids (silicon, germanium,tin) which contain four valence electrons by group V elements(phosphorus, arsenic, antimony) which contain five loosely bound valenceelectrons. One example of an n-type semiconductor is silicon with aphosphorous dopant.

In order to minimize reflection of the sunlight by the solar cell, anantireflection coating (ARC), such as silicon nitride (SiN_(x)), siliconoxide (SiO₂), alumina oxide (Al₂O₃), or titanium oxide (TiO₂), isapplied to the n-type or p-type diffusion layer to increase the amountof light absorbed into the solar cell. The ARC is typicallynon-conductive, and may also passivate the surface of the siliconsubstrate.

For silicon solar cell metallization processes, a rear contact istypically first applied to the silicon substrate. A typical processinvolves applying a back side silver paste or silver/aluminum paste toform soldering pads, followed by an aluminum paste applied to the entireback side of the substrate. Second, using an electroconductive paste, ametal contact may be screen printed onto the front side antireflectionlayer (after drying of the back side paste) to serve as a frontelectrode. This electrical contact layer on the front face or front ofthe cell, where light enters, is typically present in a grid patternmade of “finger lines” and “bus bars” rather than a complete layerbecause the metal grid materials are typically not transparent to light.The silicon substrate with printed front side and back side paste isthen fired at a temperature of approximately 700-975° C. After firing,the front side paste etches through the ARC layer, forms electricalcontact between the grid contacts and the semiconductor, and convertsthe metal pastes to metal electrodes on the light receiving surface ofthe solar cell. The back side paste is typically fired at the same timewith the front side paste, and forms electrical contact with thebackside of the silicon substrate. The resulting metallic electrodesallow electricity to flow to and from solar cells connected in a solarpanel.

To assemble a solar module, multiple solar cells are connected in seriesand/or in parallel and the ends of the electrodes of the first cell andthe last cell are preferably connected to output wiring. The solar cellsare typically encapsulated in a transparent thermal plastic resin, suchas silicon rubber or ethylene vinyl acetate. A transparent sheet ofglass is placed on the front surface of the encapsulating transparentthermal plastic resin. A back protecting material, for example, a sheetof polyethylene terephthalate coated with a film of polyvinyl fluoridehaving good mechanical properties and good weather resistance, is placedunder the encapsulating thermal plastic resin. These layered materialsmay be heated in an appropriate vacuum furnace to remove air, and thenintegrated into one body by heating and pressing. Furthermore, sincesolar cells are typically left in the open air for a long time, it isdesirable to cover the circumference of the solar cell with a framematerial consisting of aluminum or the like.

A typical electroconductive paste contains metallic particles, glassfrit, and an organic vehicle. These components are usually selected totake full advantage of the theoretical potential of the resulting solarcell. For example, it is desirable to maximize the contact between themetallic paste and silicon surface, and the metallic particlesthemselves, so that the charge carriers can flow through the interfaceand finger lines to the bus bars. The glass particles in the compositionetch through the antireflection coating layer upon firing, helping tobuild contact between the metal and the n+ type silicon. On the otherhand, the glass must not be so aggressive that it shunts the p-njunction after firing. Thus, the goal is to minimize contact resistancewhile keeping the p-n junction intact so as to achieve improvedefficiency. Known compositions have high contact resistance due to theinsulating effect of the glass in the interface of the metallic layerand silicon wafer, as well as other disadvantages such as highrecombination in the contact area. Further, glass frit is known to havewide melting temperature ranges, making their behavior stronglydependent on the processing parameters. Accordingly, electroconductivepaste compositions with improved electrical properties are desirable.

U.S. Patent Application Publication No. 2011/0308595 A1 discloses athick-film paste for printing on the front-side of a solar cell devicehaving one or more insulating layers. The thick-film paste comprises anelectrically conductive metal, and lead-tellurium-oxide dispersed in anorganic medium. The lead-tellurium-oxide is present in an amount of 0.5to 15 wt. % of solids of the paste and the molar ratio of lead totellurium is between 5/95 and 95/5. The lead-tellurium-oxide (Pb—Te—O)is prepared by mixing TeO₂ and lead oxide powders, heating the powdermixture in air or an oxygen-containing atmosphere to form a melt,quenching the melt, grinding and ball-milling the quenched material, andscreening the milled material to provide a powder with the desiredparticle size.

U.S. Pat. No. 5,066,621 discloses a sealing glass compositioncomprising, in wt. %, 13-50% lead oxide, 20-50% vanadium oxide, 2-40%tellurium oxide, up to 40% selenium oxide, up to 10% phosphorous oxide,up to 5% niobium oxide, up to 20% bismuth oxide, up to 5% copper oxideand up to 10% boron oxide and an electrically conductive formulationcomprising, in wt. %, 50-77% silver, 8-34% of a sealing glasscomposition as described previously, 0.2-1.5% resin and thixotrope and10-20% organic solvent. The '621 patent discloses tellurium oxide in apreferred range of 9-30 wt. %.

U.S. Patent Application Publication No. 2011/0192457 discloses anelectroconductive paste containing an electro-conductive particle, anorganic binder, a solvent, a glass frit, and an organic compoundincluding alkaline earth metal, a metal with a low melting point or acompound affiliated with a metal with a low melting point. The '457publication teaches the use of lead-free glass frit, for example bismuth(Bi) containing glass frit and barium (Ba) containing glass frit.

U.S. Pat. Nos. 7,736,546 and 7,935,279 disclose lead-free glass fitswith no intentionally added lead which comprise TeO₂ and one or more ofBi₂O₃, SiO₂ and combinations thereof. The patents also discloseconductive inks comprising the glass fits, and articles having suchconductive inks applied. The TeO₂ is incorporated with the one or moreof Bi₂O₃, SiO₂ in a lead-free glass matrix.

SUMMARY OF THE INVENTION

The present invention provides an inorganic reaction system for anelectroconductive paste comprising a lead containing matrix formingcomposition and tellurium oxide additive, wherein the lead containingcomposition is between 5-95 wt. % of the inorganic reaction system, andthe tellurium oxide additive is between 5-95 wt. % of the inorganicreaction system.

According to another aspect of the present invention, the inorganicreaction system comprises a lead containing matrix forming compositionbetween 10-90 wt. % of the inorganic reaction system, and telluriumoxide additive between 10-60 wt. % of the inorganic reaction system.

According to a further aspect of the present invention, the telluriumoxide additive may be one or more tellurium dioxide, tellurium trioxide,and/or any tellurium compound that would convert to tellurium oxide attemperature 200-1000° C.

According to an additional aspect of the present invention, thetellurium oxide additive is of an average particle size of less than 10μM. More preferably, the tellurium oxide additive is of an averageparticle size of less than 1 μM.

According to another aspect of the present invention, the leadcontaining matrix forming composition is a glass frit with an amorphousstructure, and may also incorporate crystalline phases or compounds.According to another aspect of the invention, the lead containing matrixforming composition comprises lead oxide. According to yet anotheraspect of the invention, the lead containing matrix forming compositioncomprises between about 10-90 wt. %, preferably about 25-85 wt. %, leadoxide. In another embodiment, the lead containing composition is alow-lead composition comprising between about 5-45 wt. %, preferablyabout 10-15 wt. %, lead oxide.

According to one more aspect of the present invention, the inorganicreaction system has a PbO:Tellurium oxide additive weight percentageratio of 95:5 to 5:95. Preferably, the inorganic reaction system has aPbO:Tellurium oxide additive weight percentage ratio of 10:1 to 1:10,and more preferably, the PbO:Tellurium oxide additive weight percentageratio is 5:1 to 1:5.

The present invention also provides an electroconductive pastecomposition comprising metallic particles, an inorganic reaction system,and an organic vehicle.

According to one aspect of the present invention, the metallic particlesin the electroconductive paste are at least one of silver, gold, copper,and nickel. According to another aspect of the present invention, themetallic particles in the electroconductive paste are silver. Accordingto another aspect of the present invention, the metallic particles inthe electroconductive paste comprise about 50-95 wt. % of the paste.

According to another aspect of the invention, the organic vehiclecomprises one or more of a binder, a surfactant, an organic solvent, anda thixatropic agent. According to another aspect of the presentinvention, the binder may be present in about 1-10 wt. % of the organicvehicle and comprises at least one of ethylcellulose or phenolic resin,acrylic, polyvinyl butyral or polyester resin, polycarbonate,polyethylene or polyurethane resins, or rosin derivatives. Thesurfactant may be present in about 1-10 wt. % of the organic vehicle andcomprises at least one of polyethyleneoxide, polyethyleneglycol,benzotriazole, poly(ethyleneglycol)acetic acid, lauric acid, oleic acid,capric acid, myristic acid, linolic acid, stearic acid, palmitic acid,stearate salts, palmitate salts, and mixtures thereof. The organicsolvent may be present in about 50-90% wt. % of the organic vehicle andcomprises at least one of carbitol, terpineol, hexyl carbitol, texanol,butyl carbitol, butyl carbitol acetate, dimethyladipate, or glycolether. The thixatropic agent may be present in about 0.1-5 wt. % of theorganic vehicle and comprises thixatropic agents known in the art.

The present invention further provides a solar cell produced by applyingthe electroconductive paste of the present invention to a silicon waferand firing the silicon wafer. According to one aspect of the presentinvention, the silicon wafer is of sheet resistance above 60Ω/□.According to another aspect of the present invention, the silicon waferis of sheet resistance above 65Ω/□. According to yet another aspect ofthe present invention, the silicon wafer is of sheet resistance above70Ω/□. According to a further aspect of the present invention, thesilicon wafer is of sheet resistance above 90Ω/□. According to anadditional aspect of the present invention, the silicon wafer is ofsheet resistance above 95Ω/□.

The present invention further provides a solar cell module comprisingelectrically interconnected solar cells made with the electroconductivepaste of the present invention.

The present invention further provides a method of producing a solarcell, comprising the steps of providing a silicon wafer, applying theelectroconductive paste of the present invention to the silicon wafer,and firing the silicon wafer according to an appropriate profile.

According to an aspect of the present invention, the silicon waferfurther comprises an antireflective coating. According to another aspectof the invention, the electroconductive paste of the present inventionis applied to the light receiving surface of the silicon wafer.

DETAILED DESCRIPTION

The present invention relates to electroconductive paste compositions asused in the manufacturing of solar cells. Electroconductive pastestypically comprise metallic particles, glass frit (an amorphous orpartially crystalline material), and an organic vehicle. While notlimited to such an application, such pastes may be used to form anelectrical contact layer or electrode on a solar cell. Specifically, thepastes may be applied to the front side of a solar cell or to the backside of a solar cell and provide the path by which conductivity occursbetween cells.

One aspect of the present invention provides an inorganic reactionsystem (IRS). The IRS of the present invention provides a delivery mediafor the metallic particles, allowing them to migrate from the paste tothe interface of the metal conductor and the semiconductor substrate.The IRS of the present invention also provides a reaction media for thepaste components to undergo physical and chemical reactions at theinterface. Physical reactions include, but are not limited to, melting,dissolving, diffusing, sintering, precipitating, and crystallizing.Chemical reactions include, but are not limited to, synthesis (formingnew chemical bonds) and decomposition, reduction and oxidation, andphase transitioning. Lastly, the IRS of the present invention acts as anadhesion media that provides the bonding between the metal conductor andthe semiconductor substrate, thereby securing reliable electricalcontact performance during the lifetime of the solar device. Althoughintended to achieve the same effects, existing glass frit compositionscan result in high contact resistance due to the insulating effect ofthe glass in the interface of the metallic layer and silicon wafer. TheIRS of the present invention acts as a delivery, reaction, and adhesionmedia, but provides much lower contact resistance and higher overallcell efficiency.

More specifically, the IRS of the present invention provides improvedOhmic and Schottky contact between the metal conductor (e.g., silver)and the semiconductor emitter (e.g., silicon substrate) in the solarcell. The IRS of the present invention is a reactive media with respectto the silicon and creates active areas on the silicon emitter thatimprove overall contact mechanisms, such as through direct contact, ortunneling. The improved contact properties provide better Ohmic contactand Schottky contact, and therefore better overall solar cellperformance.

The IRS of the present invention may comprise crystalline or partiallycrystalline materials. It may comprise various compounds including, butnot limited to, oxides, salts, fluorides, and sulfides, as well asalloys, and elemental materials.

The preferred embodiment of the present invention relates to an IRS asused in an electroconductive paste that comprises a lead containingmatrix forming composition and a tellurium oxide additive. The matrixforming composition fuses or sinters at the firing temperature of thepresent invention IRS and/or the electroconductive paste comprising anIRS according to the present invention. The matrix forming compositionmay be a glass, ceramic, or any compounds known to one skilled in theart that can form a matrix at elevated temperature. A preferredembodiment of the lead containing matrix forming composition is a leadcontaining glass frit. More preferably, a glass frit comprises leadoxide as a starting material. The lead containing matrix formingcomposition is between 5-95 wt. % of the IRS, more preferably between25-60 wt. % of the IRS. Further, the lead containing matrix formingcomposition comprises about 5-95 wt. %, preferably about 10-90 wt. %,more preferably 25-85 wt. %, and even more preferably about 45-75 wt. %,lead oxide. In another embodiment, the lead matrix forming compositionmay contain a relatively low lead content, e.g., between about 5-45 wt.%, preferably about 10-40 wt. %, and more preferably about 10-15 wt. %,lead oxide.

Used in the context of the present invention, the term additive refersto a component of the IRS that is discrete, particularly not part of amatrix forming composition. An additive is provided directly to the IRS.In the preferred embodiment, where the lead containing matrix formingcomposition is a lead containing glass frit, the tellurium oxideadditive is not a part of the lead containing glass frit.

The inclusion of the tellurium oxide additive greatly improves contactwith the semiconductor emitter, and reduces the serial resistance.Tellurium oxide as an additive performs a totally different thermaldynamic reaction on a silicon wafer comparing to lead containingtellurite glass, compounds or composition. Tellurium oxide has highreactivity with silicon. The reaction between TeO₂ and Si has a Gibbsfree energy change at 1000 K of ΔG=−140.949 Kcal/mol. The reactionbetween PbO and Si has a AG=−59.249 Kcal/mol. For Pb—Te—O glass, the AGshould be even smaller. PbO and Pb—Te—O solids have a smaller AG thantellurium oxide, which suggests lower reactivity with silicon.(Thermodynamic stability of binary oxides in contact with silicon, K. J.Hubbard and D. G. Schlom, J. Mater. Res., Vol. 11, No. 11, (1996)). Itis believed that high reactivity with the silicon wafer could result inthe formation of reactive contact points on the silicon wafer. The highreactivity of tellurium oxide may also contribute to formation ofcontact on certain high efficiency wafers with low surface dopingconcentration. Also, with tellurium oxide as an additive, the IRS systemcan be easily adapted to different glass frits and glass chemistries fora variety of electroconductive paste applications.

Incorporating tellurium oxide as an additive to the IRS also providesgreat flexibility to metallization paste formulation in industrialapplications. Instead of making Pb—Te—O solids as glass frit and usingsuch material in paste formulation, using tellurium oxide as an additiveallows the paste reactivity to be easily adjusted to meet differentSi-wafer (such as vary doping concentrations) and emitter structures.

The industrial trend for solar cell production is moving toward the fastfiring process, with very fast belt speed and “Spike” firing profiles.The controllable high reactivity of the metallization paste according tothe present invention with tellurium oxide additive is very suitable forsuch a process.

The tellurium oxide additive is between 5-95 wt. % of the IRS, morepreferably between 10-60 wt. % of the IRS. In some embodiments, thetellurium oxide additive may comprise of submicron particles having aD50 less than 1 μM. In other embodiments, the tellurium oxide additivemay be less than 10 μM in average particle size (D50).

The tellurium oxide additive is preferably tellurium dioxide (TeO₂),although tellurium trioxide (TeO₃) can also be used. In addition totellurium oxides, other tellurium-oxygen compounds can be used,including, but not limited to, tellurious acid compound, telluric acid,organic telluric compounds, and any telluric compound that would producetelluride oxide during the firing process.

In a preferred embodiment, the lead containing composition is a type ofglass frit (with an amorphous structure, and may also incorporatecrystalline phases or compounds) having lead containing compounds asstarting materials. The higher the amount of lead in the glass frit, thelower the glass transition temperature of the glass. However, higherlead amounts may also cause shunting in the semiconductor substrate,thereby decreasing the resulting solar cell's efficiency. In a preferredembodiment, lead oxide is used. More preferably, the glass frit containsabout 35-95 wt. % lead oxide, preferably about 40-85 wt. % lead oxide.

The present invention IRS can have a PbO:Tellurium oxide additive weightpercentage ratio of 95:5 to 5:95. Preferably, the PbO:Tellurium oxideadditive weight percentage ratio is 10:1 to 1:10. More preferably, thePbO:Tellurium oxide additive weight percentage ratio is 5:1 to 1:5.

Glass frits of the present invention may also include other oxides orcompounds known to one skilled in the art for making glass frits. Forexample, silicon, boron, aluminum, bismuth, lithium, sodium, magnesium,zinc, titanium, zirconium oxides and compounds. Other glass matrixformers or glass modifiers, such as germanium oxide, vanadium oxide,tungsten oxide, molybdenum oxides, niobium oxides, tin oxides, indiumoxides, other alkaline and alkaline earth metal (such as K, Rb, Cs andBe, Ca, Sr, Ba) compounds, rare earth oxides (such as La₂O₃, ceriumoxides), phosphorus oxides or metal phosphates, transition metal oxides(such as copper oxides and chromium oxides), metal halides (such as leadfluorides and zinc fluorides may also be part of the glass composition.

Lead containing glass frit can be made by any process known to oneskilled in the art. For example, glass frit components, in powder form,may be mixed together in a V-comb blender. The mixture is then heated toa very high temperature (around 120° C.) for about 30-40 minutes. Theglass is then quenched, taking on a sand-like consistency. This coarseglass powder is then milled, such as in a ball mill or jet mill, until afine powder results. Lead containing glass frit can alternativelycomprise lead oxides, salts of lead halides, lead chalcogenides, leadcarbonate, lead sulfate, lead phosphate, lead nitrate and organometalliclead compounds or compounds that can form lead oxides or salts duringthermal decomposition. In another embodiment, lead oxide may be mixeddirectly with other components of the IRS of the present inventionwithout the need of first processing the lead oxide into the form of aglass frit.

The IRS of the present invention may be created through any number ofprocesses known to one skilled in the art. For example, the IRSparticles, having an average particle size of around 0.1-10 μM (D50) aremixed from different raw IRS materials. The average particle size isdependent on the particle size of the raw IRS materials and the mixingprocess. A good mixing process should result in a well-dispersed mixtureof the IRS components.

In another example, conventional solid state synthesis may be used toprepare the IRS. In this case, raw materials are sealed in afused-quartz tube or tantalum or platinum tube under vacuum, and thenheated to 700-1200° C. The materials dwell at this elevated temperaturefor 12-48 hours and then are slowly cooled (0.1° C./minute) to roomtemperature. In some cases, solid state reactions may be carried out inan alumina crucible in air.

In another example, co-precipitation may be used to form the IRS. Inthis process, the metal elements are reduced and co-precipitated withother metal oxides or hydroxides from a solution containing metalcations by adjusting the pH levels or by incorporating reducing agents.The precipitates of these metals, metal oxides or hydroxides are thendried and fired under vacuum at 400-600° C. and fine powders of thecompounds are formed.

IRS according to the present invention may also comprise additionaladditives, which can be any oxides and compounds known to one skilled inthe art to be useful as additives. For example, boron, aluminum,bismuth, lithium, sodium, magnesium, zinc, phosphate. Other glass matrixformers or glass modifiers, such as germanium oxide, vanadium oxide,tungsten oxide, molybdenum oxides, niobium oxides, tin oxides, indiumoxides, other alkaline and alkaline earth metal (such as K, Rb, Cs andBe, Ca, Sr, Ba) compounds, rare earth oxides (such as La₂O₃, ceriumoxides), phosphorus oxides or metal phosphates, transition metal oxides(such as copper oxides and chromium oxides), metal halides (such as leadfluorides and zinc fluorides) may also be used as additives to adjustglass properties such as glass transition temperature.

Another aspect of the present invention relates to an electroconductivepaste composition which comprises metallic particles, the IRS of thepresent invention, and an organic vehicle. The preferred metallicparticles are silver, but can be any known conductive metal or mixturethereof, including, but not limited to, gold, copper, or nickel. Themetallic particles are about 50-95 wt. % of solid content of the paste,preferably about 75-95 wt. % of solid content of the paste. The IRS isabout 1-10 wt. % of solid content of the paste, preferably about 2-8 wt.%, more preferably about 5% of solid content of the paste. The amount oftellurium oxide additive may also be measure based on weight percentageof paste. Typically the tellurium oxide additive can be of 0.1-5% wt.paste. More preferably, 0.3-5% wt. of paste.

The organic vehicle may comprise a binder and a solvent, as well as asurfactant and thixatropic agent. Typical compositions of the organicvehicle are known to those of skill in the art. For example, a commonbinder for such applications is a cellulose or phenolic resin, andcommon solvents can be any of carbitol, terpineol, hexyl carbitol,texanol, butyl carbitol, butyl carbitol acetate, or dimethyladipate orglycol ethers. The organic vehicle also includes surfactants andthixatropic agents known to one skilled in the art. Surfactants caninclude, but are not limited to, polyethyleneoxide, polyethyleneglycol,benzotriazole, poly(ethyleneglycol)acetic acid, lauric acid, oleic acid,capric acid, myristic acid, linolic acid, stearic acid, palmitic acid,stearate salts, palmitate salts, and mixtures thereof. The organicvehicle is about 1-20 wt. % of the paste, preferably about 5-15 wt. % ofthe paste. The thixatropic agent is about 0.1-5 wt. % of the paste.

To form an electroconductive paste, the IRS materials are combined withelectroconductive particles, e.g., silver, and an organic vehicle usingany method known in the art for preparing a paste composition. Themethod of preparation is not critical, as long as it results in ahomogenously dispersed paste. The components can be mixed, such as witha mixer, then passed through a three roll mill, for example, to make adispersed uniform paste. In addition to mixing all of the componentstogether simultaneously, the raw IRS materials can be co-milled withsilver particles in a ball mill for 2-24 hours to achieve a homogenousmixture of IRS and silver particles, which are then combined with theorganic solvent in a mixer.

Such a paste may then be utilized to form a solar cell by applying thepaste to the antireflection layer on the silicon substrate, such as byscreen printing, and then drying and firing to form an electrode on thesilicon substrate.

The preferred embodiments as described above of the present inventionIRS system with a lead containing matrix forming composition and atellurium oxide additive and electroconductive paste made thereof aretypically applied to the light receiving surface of a silicon wafer.Typically, the present invention electroconductive paste is screenprinted over the ARC of a silicon wafer. Other application methods, suchus stenciling, may also be used to apply the electroconductive paste.However, the foregoing does not preclude incorporating the presentinvention IRS system to an electroconductive paste intended for thebackside of the silicon wafer.

Example 1

As shown in Table 1, exemplary electroconductive pastes T1-T4 wereprepared with an IRS comprising a glass frit comprising about 43% PbO(in IRS) and a number of metal oxide additives. Particularly, exemplaryelectroconductive paste T1 comprises 1.5% wt. paste of bismuth oxide(Bi₂O₃), T2 comprises 1.5% wt. paste of tellurium oxide (TeO₂), T3comprises 1.5% wt. paste of tin oxide (SnO), and T4 comprises 1.5% wt.paste of antimony trioxide (Sb₂O₃). Silver particles, in an amount ofabout 85 wt. % (of paste), and an organic vehicle, in an amount of about1-10 wt. % (of paste), were added to form the exemplary pastes.Exemplary solar cells were prepared using lightly-doped p-type siliconwafers with a sheet resistance of 80Ω/□.

TABLE 1 Metal oxide additives in electroconductive paste formulationsReference IRS Components paste T1 T2 T3 T4 Leaded Glass A 4.6 3.1 3.13.1 3.1 Glass Frit (43% PbO) Oxide Bi₂O₃ 1.5 Additives TeO₂ 1.5 SnO 1.5Sb₂O₃ 1.5 Total IRS % 4.60 4.60 4.60 4.60 4.60 in paste PbO w. % 43.48%43.48% 43.48% 43.48% 43.48% in IRS

The paste was screen printed onto the front side of silicon wafers at aspeed of 150 mm/s, using a 325 (mesh)*0.9 (mil, wire diameter)*0.6 (mil,emulsion thickness)*70 μm (finger line opening) calendar screen. Analuminum back side paste was also applied to the back side of thesilicon wafer. The printed wafer was dried at 150° C. and then fired ata profile with the peak temperature of about 750-900° C. for a fewseconds in a linear multi-zone infrared furnace. A commercial paste isused as reference.

All solar cells were then tested using an I-V tester. A Xe arc lamp inthe I-V tester was used to simulate sunlight with a known intensity andthe front surface of the solar cell was irradiated to generate the I-Vcurve. Using this curve, various parameters common to this measurementmethod which provide for electrical performance comparison weredetermined, including solar cell efficiency (Eta %), series resistanceunder three standard lighting intensities (Rs3 mΩ), fill factor (FF %).Direct measurement of contact resistance by four-probe technique isalways used by researchers, but the measurement accuracy is verydependent on sample preparation. Therefore, in the circumstance when thefinger line resistivity (usually the same silver material and firingcondition) and finger line geometry (printing related) are identical,series resistance Rs3 given by H.A.L.M IV tester can be used to evaluatethe electrical contact behavior of the conductive paste to siliconsubstrate. Generally, the smaller the Rs3, the better contact behaviorof the silver pastes. Data for the reference paste was normalized to 1.The relevant data for the experimental pastes was calculated by dividingthe appropriate measurement by the normalized reference cell data.Selected electrical performance data for exemplary pastes T1-T4 iscompiled in Table 2.

It is clear from the data presented in Table 2 that exemplary paste T2with tellurium oxide additive unexpectedly improves serial resistance,as shown through significant reduction of the Rs3 measurement, whichalso results in increased solar cell efficiency and fill factor gain.Other tested metal oxides failed to show similar beneficial effects,even though these metal oxides are also known to modify glass softeningtemperature and adjust glass flowability.

TABLE 2 Electric performance of solar cell produced usingelectroconductive paste formulations comprising metal oxide additivesReference paste T1 T2 T3 T4 Eta 1.0000 1.0006 1.0077 1.0036 0.8349 FF1.0000 1.0052 1.0081 1.0040 0.8366 Rs3 1.0000 0.9771 0.9462 1.05484.9133

Example 2

A number of additional exemplary pastes E1-E22 were prepared with fourglass frits A (43% PbO), B (60% PbO), C (67% PbO), D (73% PbO) andvarious amounts of tellurium oxide additive. Details of the exemplarypaste formulations and PBO and TeO₂ content and weight ratio of eachexemplary paste is represented in Table 3. Exemplary solar cells, usingmono-crystalline or polycrystalline silicon wafers with varying sheetresistance, were prepared by the process set forth in Example 1 above.More specifically, selected electrical performance data of solar cellsprepared with multi-crystalline silicon wafer type 1A (sheet resistanceof 70Ω/□) is present in Table 4, electrical performance data of solarcells prepared with multi-crystalline silicon wafer 1B (sheet resistanceof 95Ω/□) is present in Table 5, electrical performance data of solarcells prepared with mono-crystalline silicon wafer type 2 (sheetresistance of 65Ω/□) is present in Table 6, electrical performance dataof solar cells prepared with mono-crystalline silicon wafer type 3(sheet resistance of 90Ω/□) is present in Table 7, electricalperformance data of solar cells prepared with multi-crystalline siliconwafer type 4 (sheet resistance of 60Ω/□) is present in Table 8,electrical performance data of solar cells prepared withmono-crystalline silicon wafer type 5 (sheet resistance of 60Ω/□) ispresent in Table 9, electrical performance data of solar cells preparedwith multi-crystalline silicon wafer type 6 (sheet resistance of 70Ω/□)is present in Table 10, electrical performance data of solar cellsprepared with mono-crystalline silicon wafer type 7 (sheet resistance of70Ω/□) is present in Table 11, and electrical performance data of solarcells prepared with multi-crystalline silicon wafer type 8 (sheetresistance of 65Ω/□) is present in Table 12. All data are normalizedagainst reference paste on silicon wafer type 1A. Particularly, relativeefficiency, relative fill factor, and relative Rs3 measurements for allexemplary pastes E1-E22 on all other silicon wafer types are normalizedagainst efficiency, fill factor, and Rs3 measurements of reference pasteon silicon wafer type 1A, respectively.

TABLE 3 IRS formulations with tellurium oxide additive (Component wt. %in Paste) IRS IRS Compo- Reference Components nents paste E1 E2 E3 E4 E5E6 E7 Leaded Glass A 4.6 3.1 2.9 2.7 Glass Frit (43% PbO) Glass B 2.09(60% PbO) Glass C 3.1 3.1 3.1 (67% PbO) Glass D (73% PbO) TeO₂ 1.5 1.51.5 2.51 1.5 1.2 1 (wt. % paste) Total IRS 4.6 4.6 4.4 4.2 4.6 4.6 4.34.1 (wt. % paste) PbO w. % in 43.48% 43.73% 45.72% 47.89% 26.88% 45.00%48.14% 50.49% IRS TeO₂ w. % in — 32.61% 34.09% 35.71% 54.57% 32.61%27.91% 24.39% IRS PbO/TeO₂ — 0.90 0.88 0.86 0.22 0.93 1.24 1.57 ratio,weight IRS IRS Compo- Components nents E8 E9 E10 E11 E12 E13 E14 E15Leaded Glass A Glass Frit (43% PbO) Glass B (60% PbO) Glass C 2.79 2.482.79 2.79 3.1 2.66 2.48 2.61 (67% PbO) Glass D (73% PbO) TeO₂ 0.7 0.80.9 0.6 0.5 0.9 0.6 0.9 (wt. % paste) Total IRS 3.49 3.28 3.69 3.39 3.63.555 3.08 3.51 (wt. % paste) PbO w. % in 53.38% 50.49% 50.49% 54.96%57.50% 52.41% 53.77% 53.08% IRS TeO₂ w. % in 20.06% 24.39% 24.39% 17.70%13.89% 25.32% 19.48% 25.64% IRS PbO/TeO₂ 2.13 1.57 1.57 2.56 3.57 1.552.22 1.54 ratio, weight IRS IRS Compo- Components nents E16 E17 E18 E19E20 E21 E22 Leaded Glass Glass A Frit (43% PbO) Glass B (60% PbO) GlassC 2.03 2.17 1.96 (67% PbO) Glass D 3.2 2.7 2.2 1.7 (73% PbO) TeO₂ 0.50.7 0.7 1 1 1 1 (wt. % paste) Total IRS 2.53 2.87 2.66 4.2 3.7 3.2 2.7(wt. % paste) PbO (wt. % 57.27% 50.49% 54.47% 55.24% 50.16% 43.50%34.37% in IRS) TeO₂ (wt. % 19.76% 24.39% 26.32% 23.81% 27.03% 31.25%37.04% in IRS) PbO/TeO₂ 2.33 1.57 1.53 1.77 1.35 0.96 0.58 ratio, weight

TABLE 4 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 1A Reference paste E1 E19 E20 E21 E22 Eta 1.00001.0056 1.0134 1.0202 1.0212 1.0111 FF 1.0000 1.0131 1.0144 1.0137 1.01891.0132 Rs3 1.0000 0.7963 0.6804 0.6748 0.8198 0.8076

TABLE 5 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 1B Reference paste E16 E17 Eta 1.0053 1.02011.0272 FF 0.9808 0.9973 1.0032 Rs3 1.4524 0.8204 0.7654

TABLE 6 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 2 Reference paste E1 E2 E3 E4 Eta 1.0378 1.08091.0856 1.0773 1.0773 FF 0.9912 1.0242 1.0279 1.0119 0.9901 Rs3 0.95260.4839 0.4938 0.6154 0.8897

TABLE 7 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 3 Reference paste E1 E2 Eta 0.8040 1.1063 0.9976FF 0.7360 1.0130 0.9105 Rs3 4.1267 0.6909 1.9430

TABLE 8 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 4 Reference paste E10 E11 E12 E13 E14 E15 Eta0.9787 0.9876 0.9900 0.9911 0.9841 0.9900 0.9876 FF 0.9926 1.0097 1.00910.9966 1.0087 1.0083 1.0099 Rs3 0.7915 0.6491 0.6647 0.8200 0.68470.6676 0.6636

TABLE 9 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 5 Reference paste E5 E6 E7 E8 E10 E15 Eta 0.96690.9805 0.9876 0.9876 0.9852 0.9935 0.9935 FF 0.9917 1.0097 1.0105 1.01171.0092 1.0097 1.0099 Rs3 1.0455 0.6733 0.6488 0.6597 0.6985 0.64910.6636

TABLE 10 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 6 Reference paste E5 E6 E7 E8 E9 Eta 0.9693 0.98290.9882 0.9947 0.9953 0.9811 FF 0.9900 1.0157 1.0154 1.0156 1.0174 1.0140Rs3 1.1562 0.5718 0.5524 0.5713 0.5595 0.5728

TABLE 11 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 7 Reference paste E16 E17 E18 Eta 1.0915 1.11751.1163 1.1151 FF 1.0144 1.0329 1.0345 1.0333 Rs3 0.7726 0.5379 0.53660.5337

TABLE 12 Electric performance of solar cell produced usingelectroconductive paste formulations comprising tellurium oxide additiveon silicon wafer type 8 Reference paste E11 E16 Eta 0.9658 0.9752 0.9929FF 0.9689 0.9946 0.9987 Rs3 1.1484 0.8387 0.9133

As shown in Tables 4-12, exemplary pastes E 1-E22 are shown to producesolar cells with overall improved serial resistance as evidenced by theRs3 measurements. The improvement in serial resistance also contributesto improved overall solar cell performance. For all types of siliconwafers tested, the exemplary pastes outperform the reference paste interms of relative efficiency and/or fill factor. The most dramaticimprovement over the commercial reference paste is shown throughexemplary pastes comprising tellurium oxide additives according to thepresent invention with high sheet resistance silicon wafers, e.g., type3 wafer (mono-crystalline with 90Ω/□ of sheet resistance). The referencepaste performed poorly with this type of silicon wafer, providing poorserial resistance and overall subpart solar cell performance. Theexemplary pastes E1 and E2, comprising the same type of PbO containingglass frit as the reference paste, showed drastically improvedperformance over the reference paste, providing very good serialresistance measurements and superior solar cell overall performance.

The electrical performance data for the reference paste shown in Table4-12 also clearly demonstrates one persisting difficulty for themetallization paste technology. The same reference paste, applied usingexactly the same screen printing and firing procedure on a number ofsilicon wafers of varying sheet resistance, produced solar cells ofdisparaging performance characteristics. For example, the relevantefficiency of the cells for the reference paste in Table 4-12 is from0.8040 to 1.0915, which is a difference of over 28%. As an industrialprocess, such large variance is not acceptable. Paste composition thusmust to be modified for each type of silicon wafer to achieve optimalperformance of the resulting solar cells. The modification process istypically time-consuming, since an electroconductive paste comprises anumber of components, allow of which may need to be optimized.

The present invention incorporates a tellurium oxide additive. As anadditive, and not part of the matrix forming composition, the amount ofthe tellurium oxide additive can be readily adjusted for different typesof silicon wafer. It is believed that reactivity of the IRS system canbe fine-tuned by adjusting the tellurium oxide additive. The datapresented in Table 4-12 shows that by adjusting lead containing matrixforming compositions and tellurium oxide additive, optimal performancecan be expected from all the tested silicon wafers. For exemplary pastesE1-E22, types and amounts of lead containing glass frit and amounts oftellurium oxide are adjusted. As shown above, the resulting solar cellsoutperform the reference paste in terms of relative efficiency and/orfill factor (Table 4-12). This is particularly true for silicon waferswhere the reference paste fails to perform (Table 7).

These and other advantages of the present invention will be apparent tothose skilled in the art from the foregoing specification. Accordingly,it will be recognized by those skilled in the art that changes ormodifications may be made to the above described embodiments withoutdeparting from the broad inventive concepts of the invention. Specificdimensions of any particular embodiment are described for illustrationpurposes only. It should therefore be understood that this invention isnot limited to the particular embodiments described herein, but isintended to include all changes and modifications that are within thescope and spirit of the invention.

What is claimed:
 1. An inorganic reaction system for an electroconductive paste comprising a lead containing matrix forming composition and a tellurium oxide additive, wherein the lead containing composition is between 5-95 wt. % of the inorganic reaction system, and the tellurium oxide additive is between 5-95 wt. % of the inorganic reaction system.
 2. The inorganic reaction system of claim 1, wherein the lead containing matrix forming composition is between 10-90 wt. % of the inorganic reaction system, and the tellurium oxide additive is between 10-60 wt. % of the inorganic reaction system.
 3. The inorganic reaction system of claim 1, wherein the tellurium oxide additive comprises one or more of tellurium dioxide, tellurium trioxide, and any tellurium compound that would convert to tellurium oxide at temperature 200-1000° C.
 4. The inorganic reaction system of claim 1, wherein the tellurium oxide additive is of an average particle size of less than 10 μM.
 5. The inorganic reaction system of claim 1, wherein the tellurium oxide additive is of an average particle size of less than 1 μM.
 6. The inorganic reaction system of claim 1, wherein the lead containing matrix forming composition is a glass frit.
 7. The inorganic reaction system of claim 1, wherein the lead containing matrix forming composition comprises lead oxide.
 8. The inorganic reaction system of claim 7, wherein the lead containing matrix forming composition comprises between about 10-90 wt. %; lead oxide.
 9. The inorganic reaction system of claim 7, wherein the lead containing matrix forming composition comprises between about 5-45 wt. %; lead oxide.
 10. The inorganic reaction system of claim 7, wherein the inorganic reaction system has a lead oxide:Tellurium oxide additive weight percentage ratio of 95:5 to 5:95.
 11. An electroconductive paste composition comprising: metallic particles; an inorganic reaction system as in claim 1; and an organic vehicle.
 12. The electroconductive paste as in claim 11, wherein the metallic particles are at least one of silver, gold, copper, and nickel.
 13. The electroconductive paste as in claim 11, wherein the metallic particles are silver.
 14. The electroconductive paste as in claim 11, wherein the metallic particles are about 50-95 wt. % of solid content of the paste. 15-22. (canceled)
 23. A solar cell produced by applying an electroconductive paste according to claim 11 to a silicon wafer and firing the silicon wafer.
 24. (canceled)
 25. (canceled)
 26. The solar cell of claim 23, wherein the silicon wafer is of sheet resistance above 70Ω/□.
 27. The solar cell of claim 23, wherein the silicon wafer is of sheet resistance above 90Ω/□.
 28. The solar cell of claim 23, wherein the silicon wafer is of sheet resistance above 95Ω/□. 29-32. (canceled)
 33. The inorganic reaction system of claim 5, wherein the glass frit is at least partially crystalline.
 34. The inorganic reaction system of claim 8, wherein the lead containing matrix forming composition comprises between about 25-85 wt. % lead oxide.
 35. The inorganic reaction system of claim 9, wherein the lead containing matrix forming composition comprises between about 10-15 wt. % lead oxide.
 36. The inorganic reaction system of claim 7, wherein the inorganic reaction system has a lead oxide:Tellurium oxide additive weight percentage ratio of 10:1 to 1:10.
 37. The inorganic reaction system of claim 7, wherein the inorganic reaction system has a lead oxide:Tellurium oxide additive weight percentage ratio of 5:1 to 1:5. 